Given that the sum of the first n terms of the arithmetic sequence {an} is Sn = 2n & # 178; - 10N, find the values of (1) A1 and A3; (2) find A5 + A6 + A7 + A8; (3) find its general term formula and judge whether it is arithmetic sequence

Given that the sum of the first n terms of the arithmetic sequence {an} is Sn = 2n & # 178; - 10N, find the values of (1) A1 and A3; (2) find A5 + A6 + A7 + A8; (3) find its general term formula and judge whether it is arithmetic sequence

a(1) = s(1)= 2-10= -8.
s(n) = 2n^2 - 10n,
a(n+1) = s(n+1) - s(n) = [2(n+1)^2 - 10(n+1)] - [2n^2 - 10n] =2(2n+1) - 10 = 4n -8,
a(n) = 4(n-1) - 8 = 4n - 12.
{a (n) = 4 (n-1) - 8} is an arithmetic sequence with the first term of a (1) = - 8 and the tolerance of 4
a(1) = -8,a(3) = 4*3 - 12 = 0.
a(5) + a(6) + a(7)+a(8) = 4[5+6+7+8] - 12*4 = 4[5+6+7+8-12] = 4*14 = 56